This invention relates to a painting frame provided with adjustable joints for the purpose of adjusting the tension on a piece of canvas attached thereto. More particularly, the invention concerns a corner joint which allows the depth of engagement of the frame side members within the joint to be varied. In a supporting frame whereto a canvas piece is affixed such that a painting or picture may be painted thereon, the problem arises of ensuring a uniform or even tension for the canvas, the latter being fastened to the frame by means of nails or tacks. The expert in the art, as well as the user of such articles, is well aware of the importance of the stretching operation in order to provide satisfactory results and a proper preservation of a painting. In order to facilitate such an operation, several forms of joint have been developed and applied in the past which, however, have in common the configuration of the frame side members or strips, having a 45.degree. bevel at their ends, with one or more projecting lugs, and respectively one or more recesses, for joint engagement with the lugs of the contiguous side member or strip. The depth of insertion of each frame strip into a contiguous strip is made adjustable in a longitudinal direction by the interposition, within the recesses, of a wedge in each strip and at each end, there being provided eight adjusting wedges, i.e. two for each frame corner. In very large frames, further transverse or cross stretching strips may be inserted at intermediate positions between opposite sides. This known frame adjusting method, directed to evenly stretching the canvas, has some shortcomings. Both the lugs, or joint tongues, and their respective recesses or sockets have to be comparatively large and thin, while at the same time retaining their strength requirements. Their construction is thus a complex work, and requires several working steps. Furthermore, not all the kinds of wood lend themselves to such critical processing, and a selection of costly high quality woods becomes necessary. A further finishing step is then required for the strips in order to prevent the canvas from assuming antiaesthetic creased appearance due to its pressing against the strip inner corner edge; it is thus necessary to bevel inwardly the strip face whereon the canvas rests and to round off its inner corner edge. This operation also effects the cost of a frame of this type.